High frequency isolator



May 19, 1959 v H. SUHL 2,887,665

A HIGH FREQUENCY ISOLATOR Filed Dec. 31, 1953 Fla.

HA L E FE 1' MATER IAI.

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m VENTOR H. SUHL BVWJ:

A T TORNE Y United States Patent Ofiice HIGH FREQUENCY ISOLATOR HarrySuhl, lrvington, N.J., assignor to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York ApplicationDecember- 31, 1953, Serial No. 401,530

6 (Jlaims. (Cl. 333-24) This invention relates to electromagnetic wavetransmission devices and more particularly to such devices havingnonreciprocal transmission properties.

An object of this invention is to supply a wave energy isolator which isuseful atfrequencies much be'low microwave frequencies.

Another object is to provide a :coaxial :cable type isolator.

A nonreciprocal wave transmission device .ison'e which causes theattenuation or phase shift of a wave propagating through it in a givendirection to :be different from the attenuation or phase shift of asimilar wave "propagating in the reverse direction through the device.While numerous examples of such devices are .known, one type which canbe :cited .as an example is the microwave polarization rotating deviceand coupling network shown in :United States Patent :2,'6'44;930 tokLuh'r's et a l. The arrangement shown in this patent 'utilizes therotationof a polarized electromagnetic 'wavepropa'gating through aFaraday effectimaterial, such as aferrite, to obtain nonreciprocaltransmission characteristics. However, because of the nature of theFaraday etf'ect materials at present available and the structuralrequirements of the presently known ways of utilizing them, the range ofapplication of the above mentioned type of isolator is not easilyextended to frequencies much below microwave frequencies of the order of1000 'me'gacycles, nor to structures which can be connected directly incoaxial cable transmission lines. The present'in'vention seeks to supplya device which can be used at frequencies much lower than microwavefrequencies and which provides nonreciprocal transmission directly of acoaxial "cable mode electromagnetic wave.

.The operation or mode of action of the present invention depends uponthe nan effect 'rota'ti'on of an electromagnetic wave as distinguishedfrom the Faraday effect rotation. As in result certain importantstructural differences exist between the above prior art device and thefollowing illustrative embodiments of this invention. One of thesedifferences is that the steady magnetic biasing field in the former isapplied parallel to the axis of wave transmission whereas in the latterit is applied circumferentially thereof. Other important differenceswill become apparent hereinafter.

The Faraday effect produced by' 'cert'ain magnetic materials is believedto be caused by the gyroscopic precession of spinning electrons in thematerials whereas the .species of Hall effect utilized in the.presentfinvention is caused by the nonreciprocal motion ofelect-ronswithin a material subject 'to crossed electric and magnetic fields. Fora comprehensive explanation of Faraday eifect the reader is referred toan article appearing in the November 1953 issue of the Bell SystemTechnical Journal beginning on page 1333, entitled Ferrites (in:Microwave Applications by J. :Rowen. However, because of its closeconnection with the present invention a short lexplanation of Halleffect will be given here. Briefly, Hall efiect is the nonreciprocalchange in current flow through 2,887,665 Patented May 19, 1959 amaterial which occurs when a magnetic field not parallel to the currentflow is applied to the material. If i represents the current vector, Ethe electric field vector, and H the magnetic field vector, then thecurrent fiowing through a Hall effect material is where o' is theconductivity of the material and at is the Hall constant related to theHall coefiicient R as follows: a=R 0' Assuming that the magnetic fieldis unvarying and aligned along the Z axis, that there is no Z componentof electric field E and that E varies sinusoidally, the x and ycomponents of current are ZocH Explained in other terms, Equations 2 and3 show that the effective dielectric constant for one direction oftransmission through the Hall effect material is cliiferent from theeffective dielectric constant for the opposite direction oftransmission. Accordingly, the losses in the two ilirection-s aredifferent. The fact that there is a difference in loss is used toadvantage in the following illustrative embodiments.

in accordance with the present invention in one specific embodimentthereof, indium-antimony, a Hall eifect material, is substituted for aportion of a length of the center conductor of a coaxial cable and ismagnetized in such a way that the transmission loss of energypropagating in one direction through the cable is substantiallydilferent from the loss in the opposite direction. Further details ofthis and other embodiments of the invention will best be learned,however, from a consideration of the following description given inconnection with the drawings of these embodiments.

In the drawings:

"Fig. 1 shows a longitudinal cross'section of an illustrative embodimentof the invention in which a portion of a coaxial cable has a length ofits center conductor made of Hall effect material;

Fig. 2 shows a longitudinal cross section of a second illustrativeembodiment in which a length of the center conductor of a coaxial cableis Wound as a helix and is surrounded by Hall effect material; and

Fig. 3 shows a side View of another embodiment in which Hall eifectmaterial is placed in field coupling relation to a slow Wave propagatingcircuit.

Referring now in detail to the drawings, the illustrative embodiment ofthe invention shown in Fig. l is a coaxial cable type isolator 18. Thisisolator consists of a coaxial cable ill in which a length of its innerconductor 12 partially comprises Hall effect material 13. This materialmay be selected from any of the materials showing Hall effect, but ispreferably a eutectic alloy of indium-antimony, a material showing theelfect to a A convenient way of doing so in the instant embodiment is bypassing a direct current from schematically shown source 15 throughconductor 14 which is aligned with the axis of cable 11. The resultingmagnetic field will lie circumferentially around this axis and willpermeate material 13. If the plane of the paper upon which the drawingsappear is taken as the xy plane with the x coordinate parallel to theaxis of cable 11 and the z coordinate into the paper as indicated inFig, 1, then the resulting electric vectors E at the upper and lowerlongitudinal tangents of the surface of material 13 will be polarized inthe xy plane and the magnetic field generated by the current in wire 14will be perpendicular to the xy plane taken through these tangents. Adiagram of the upper resulting electric vector lying in the xy plane isshown in Fig. l. The components of this vector are E which depend uponthe voltage difference between the inner and outer conductors of cable11 and E which depend upon the conduction current flowing in material13. From symmetry it is readily seen that the resulting electric vectorsat other points around the circumference of material 13 are alsoperpendicular to the circular magnetic field. For a wave propagatingfrom left to right through isolator 10, circularly polarized electricvectors E will rotate in one direction while for a wave propagating fromright to left, they will rotate in the opposite direction. Accordingly,the difference between the transmission losses in the two directionswill be proportional to ZaH It should be understood in this regard thatthe total difference in loss depends upon factors including the kind ofHall effect material used, the length and amount of this material andthe strength of the current flowing through wire 14. Any or all of thesefactors may be chosen to produce the loss desired.

Fig. 2 shows a modification of the isolator of Fig. 1. Here a length 21of the inner conductor of coaxial cable 22 is wound as a helix and hassurrounding it Hall effect material 23 which should preferably be aeutectic alloy of indium-antimony. Winding the inner conductor for partof its length into a helix increases the axial component of the electricfield and thereby enhances the nonreciprocal transmission property ofthe isolator but at the same time this causes an undesirable impedancemismatch not present in the structure of Fig. 1. A circumferentialmagnetic field permeating material 23 may be applied by any convenientmeans such as a current carrying wire similar to wire 14 in Fig. 1. Theprinciple of operation of the embodiment of Fig. 2 is substantially thesame as that of isolator 10.

Fig. 3 is another illustrative embodiment of the invention wherein Halleflect material 30, which may be the same kind as material 13 disclosedabove with respect to Fig. 1, is positioned in close proximity to a wavepropagating helix 31. A magnetic field aligned so that the vectorproduct E H in Equation 1 is non-zero should be applied throughout theactive volume of material 30. The thickness 2? and the length and widthof this material are not critical and may be chosen according to theamount of directivity in transmission that is desired. Thickness t,however, should usually be at least as thick as the skin depth otpenetration of electromagnetic waves into material 30 at the frequencyof operation. Material 30 should be placed as close as convenient to thesurface of helix 31 but this spacing is likewise not critical. Theprinciple of operation of this arrangement is substantially the same asthat of isolator 10. In general, in the embodiment of Fig. 3, any slowwave circuit, such as those disclosed in United States Patent 2,659,817to C. C. Cutler, which generates a strong axial component A. of electricfield may be used in place of helix 31. The configuration of material 30may be modified accordingly.

The above embodiments are presented in illustration and not inlimitation of the invention. Various changes and modifications in theseembodiments will occur to those skilled in the art and these changes andmodifications may be made without departing from the spirit or scope ofthe invention as set forth. It should be noted in this connection thatnonreciprocal transmission can be obtained with Hall effect material bythe electric field displacement method in a way similar to that outlinedin the copending application of S. E. Miller, Serial No. 362.193, filedJune 17, 1953.

What is claimed is:

1. In combination, a coaxial cable having an inner and an outerconductor which are conductively separate, Hall efiect materialpositioned within the path of wave energy guided by said cablesymmetrically with respect to each of two normal planes passing throughthe center of both of said conductors, and means for establishing acircular magnetic biasing field coaxial with the conductors I of saidcable and permeating said Hall etfect material.

2. The combination of elements as in claim 1 in which a length of saidinner conductor partially comprises said Hall efiect material. i

3. The combination of elements as in claim 1 in which a length of saidinner conductor is wound as a helix and said Hall effect material ispositioned symmetrically with respect to said helix.

4. The combination of elements as in claim 1 in which said Hall effectmaterial is a eutectic alloy of indiumantimony.

5. Nonreciprocal electromagnetic wave energy transmission meansincluding in combination, a source of electromagnetic wave energycharacterized by solely trans-- verse electric field components, dualconductor coaxial wave guiding means adapted to support said wave energyconnected thereto, and a polarized element of Hall effect materiallongitudinally extending within said means and having a transverseextent sufiicient to coact with a substantial number of said transversecomponents whereby said components are modified and appear to retate inlongitudinal planes within said material, said rotation being oppositein sense for opposite directions of propagation of wave energy throughsaid means, said polarization being effected by magnetic biasing meansproducing a constant field normal at all points to said longitudinalplanes containing said rotating electric field vectors.

6. A nonreciprocal electromagnetic wave energy component comprising aconductive member having an axis, at least one conductive boundarycoaxial with and electrically insulated from said conductive member, acylinder of Hall effect material surrounding the conductive member ofsaid component and extending in the direction of propagation of waveenergy therethrough, and means for establishing lines of constantmagnetic intensity which permeate said element and extend in anencircling fashion about said conducting member.

References Cited in the file of this patent UNITED STATES PATENTS2,532,157 Evans Nov. 28, 1950 2,643,297 Goldstein June 23, 19532,647,239 Tellegen July 28, 1953 2,649,574 Mason Aug. 18, 1953 2,777,906Shockley Jan. 15, 1957 OTHER REFERENCES Publication I: Montgomery,Technique of Microwave Measurements, vol. 11, M.I.T. Radiation Lab.Series, published 1948, McGraw-Hill, pg. 197 relied on.

' HMA

